By the margin of a single vote, the Maryland House of Delegates today failed to approve the proposed D.C. voting rights amendment, handing at least a temporary setback to supporters of the measure.

The amendment's sponsors, however, remained confident they could muster the one necessary vote when the measure comes up for reconsideration Tuesday.

Apparently stunned by the sudden defeat, sponsors explained that three of their allies were absent from today's session, while several others defected from their ranks at the last moment.

"We've got to get all out people here next time," said Del. Nathaniel Exum (D-Prince George's), one of the sponsors.

Other supporters are not so optimistic, pointing out that many delegates promised to vote for the measure before sampling the opinion of their constituents and hearing opposing views.

"A lot of people committed for it and then got negative feedback back home, and now they're (wavering)," said Del. Robert Redding, using a hand gesture to show that the vote could now go either way.

The amendment -- which would give the District full voting rights in Congress if ratified by 38 state legislatures -- had led a charmed life in the Maryland legislature until today's setback.

As expected, the Senate gave final passage to an identical proposal today -- several hours after the House defeat. The House committee that studied the measure approved it by a margin of 18 to 2.

Once it reached the House floor, however, the amendment began running into obstacles. The problems that the measure has encountered illustrate in several ways how the Maryland General Assembly works.

First, the measure does not have the support of House leaders who avoided taking a united position on it. In the Senate, which handily passed the measure, almost every committee chairman backed it.

Then, the amendment's chief sponsor in the House is Del. Charles Blumenthal, a Democrat from Prince George's, who annoys many of his colleagues by his persistent nagging over technical points.

Blumenthal and his allies were chided after today's session for allowing the amendment to come up for a vote without making sure they had enough support to win approval.

"I would've asked for a delay," said Ed Lamon, a lobbyist for the AFLCIO, which strongly supports the measure. "We haven't had enough time to work on the House."

Ratification proponents may also have underestimated the strength of conservatives in the House, who normally can unite large and disparate blocs of votes against legislation they appose.

Legislators who oppose the move to make Maryland the fourth state to ratify the amendment have been aided in recent weeks by the lobbying efforts by national conservative groups, including the America Legislative Exchange Council.

Today's vote tally shows virtually solid opposition from conservative Anne Arundel County and rural quarters of the Eastern Shore, Western and Southern Maryland. A majority of delegates from conservative Baltimore County also voted "no."

Nearly all of the support for the amendment came from Baltimore City and from the two Washington suburbs -- Prince George's and Montgomery counties whose delegates put aside fears of a commuter tax to push for the measure.

The debate preceding today's vote reiterated the same points that have been aired in past hearings -- proponents basing their arguments on human rights and opponents charging that giving two U.S. senators to the District violates the U.S. Constitution, which restricts that privilege to states.